Despite increased glycemic control and use of renin-angiotensin system inhibitors, diabetic nephropathy remains a leading cause of end stage renal disease. The fibrogenic cytokine, transforming growth factor-[unreadable] (TGF-[unreadable]), is a key molecular factor in the pathogenesis of diabetic nephropathy. The expression of TGF-[unreadable] and its activity are increased in diabetes. TGF-[unreadable] is expressed as a biologically latent molecule that must be converted to its active form in order to induce fibrogenic effects. This activation step represents a major point of regulation of TGF-[unreadable] bioactivity. We identified thrombospondin 1 (TSP1) as the molecular regulator of TGF-[unreadable] activation in diabetes. TSP1 protein expression is increased by mediators of diabetic nephropathy such as glucose and angiotensin II. In vitro studies and a rat model of STZ-induced diabetes with hypertension showed that antagonism of TSP1-dependent TGF-[unreadable] activation by a four amino acid peptide (LSKL) blocked glucose and angiotensin II stimulation of TGF-[unreadable] activity, extracellular matrix production, and prevented and reversed myocardial fibrosis. The LSKL peptide when administered by intraperitoneal injection is an effective antagonist of TGF-[unreadable] -dependent fibrosis in this model and in other non-diabetic models of renal and hepatic fibrosis. The LSKL peptide represents an effective therapeutic strategy for treatment of diabetic nephropathy: this peptide has the unique advantage of selectively inhibiting only the pathogenic increases in TGF-[unreadable] activity due to TSP1-mediated activation. This distinguishes LSKL from other strategies for inhibiting TGF-[unreadable] action, which do not discriminate between homeostatic levels and pathologic excesses of TGF-[unreadable] activity. In these studies, we will test the hypothesis that administration of the LSKL peptide improves renal function and attenuates renal fibrosis by blocking activation of TGF- [unreadable] in a newly developed genetic mouse model (129/SvEv Ins2 Akita) of type 1 diabetes, which has significant proteinuria and mesangial sclerosis. In Specific Aim 1, mice will receive thrice weekly i.p. injections of LSKL or control (LSAL) peptide over a 15 week period and renal function, morphology, and TGF-[unreadable] signaling will be evaluated. Specific Aim 2 will determine whether a combined therapeutic approach that targets both the angiotensin II type 1 receptor and TSP1 has increased benefit. Importantly, this proposal will address whether homeostatic functions of TGF-[unreadable] are compromised by antagonism of TSP1-dependent activation. Specific Aim 3 will address whether blockade of TSP1-activated TGF-[unreadable] has deleterious effects on homeostatic functions of TGF-[unreadable] with respect to systemic histopathology, tumor incidence, immune cell profile, and dermal wound healing. These studies will help establish the utility of this peptide antagonist of TSP1-dependent TGF-[unreadable] activation as a novel therapeutic for diabetic nephropathy.